Introduction

In this lecture we shall seek to explore the methods that are used to connect remote computers to each other.

We shall compare the methods of transmission that are used in data communication systems.

We will then look at the concept of bandwidth and see the comparative bandwidths offered by guided and unguided transmission methods.

We will see and compare several transmission techniques, serial, parallel, baseband and broadband.

We will introduce the concept of multiplexing.  

Communication Methods

All businesses need to have access to and store large amounts of information, much of which is constantly changing and as such needs to be constantly updated. Networking is the technology that is making this possible. Now two or more business sites may be thousands of miles away yet will seem to be next door. Traditionally data processing was done centrally on a mainframe but now it is more common to find several smaller computers and dumb terminals connected within a network.

Data are transmitted within computers and for very short distances via parallel connections, allowing the fastest data transfer rate that is possible. This is not practical over long distances owing to the large number of individual wires that would be required for each connection that would make the cost of the link extremely expensive. There is also the problem of skew, where different signal components on different cables travels at different speeds, becoming a problem once the parallel connection becomes long (i.e. over ~5 metres). As a consequence, most long-distance data links are made using serial transmissions. By virtue of being transmitted in a serial manner i.e. bit by bit, the measurement of data transfer speeds is measured in bits per second.

The data travelling within the computer on the data bus is in parallel format i.e. transmitted 8, 16, 32 or 64 bits at a time and as such needs to be converted to serial format to be transmitted. This can be achieved in one of two ways:

Data can be sent to a converter chip mounted on the motherboard, called a Universal Asynchronous Receiver Transmitter (UART) which then sends the converted data to the serial port.

Data can be sent to a converter chip that is mounted on a modem card fitted in an expansion slot inside the computer. The serial data is directly used by the rest of the modem card's electronics.

Analogue and digital transmission

Data may be sent as pulses as within a computer or as a continuous signal e.g. non-digital television. The actual speed of data transmission depends upon the nature of the data, the physical means used to carry it over and the processing or adjustments required along its transmission path and at its destination.

Analogue signals are continuous i.e. they may vary in frequency and amplitude, taking any permissible value. When the amplitude of an analogue signal falls over distance, an amplifier (or set of amplifiers) is

used to bring the signal back to strength but this also involves amplifying any noise that has been introduced by the line. Unfortunately cascaded amplifiers amplify the noise on the line too, eventually making the signal unintelligible for recovery. Transmissions involving sharp transitions of signal level occupy higher bandwidth than those changing at a slower rate do. (Look at the Fourrier transform of a square wave pulse). Despite this problem, analogue signals carry better over long distances than digital signals.

Digital signals are those which vary between predefined discrete values. Digital signals may pick up noise during transmission but the noise may be removed by a repeater and the signal is re-transmitted afresh. Repeaters are used to recover the stream of 1s and 0s and they are retransmitted afresh with increased power. Digital signals do not carry well over long distances on electrical lines.

Practical Links

Leased line

Here, BT or Mercury or another communication network provider will lease a permanent dedicated cable link between two stations so there will be no waiting for a call link to be established. These lines are of high quality, providing few errors and are usually fast. As the person/ organisation who has leased the line has sole use of the link and it is dedicated, there are no call set-up times and messages arrive in the order that they were sent. There is no need for routing decisions to be made once the service provider has set up the link. This is the type of connection that a LAN will use to access the outside world.

Dial-up line

With a dial-up line, the user shares a network of existing cabling and switching apparatus. These lines are not exclusive but are cheaper since they use shared assets. Unlike a leased line, the user may sometimes find that the line (or network switching equipment) is busy. Dial-up lines suffer from poorer quality than leased lines and produce a larger number of errors per unit time, leading to repeat transmissions and since the user pays per unit time, error time costs money.

With a dial-up line, the user needs a modem (modulator/ demodulator) to convert the computer's digital signals to analogue signals so that they can pass down the twisted pair to the local exchange. The links from local exchanges are almost exclusively twisted pair and technologies such as ADSL are an attempt to squeeze as much information down the cable as possible per unit time.

Bandwidth

The absolute bandwidth of a signal is the width of the range of frequencies that it occupies. More commonly, bandwidth is generally the term used to describe effective bandwidth, that being the band or range of frequencies within which most of the signal energy is confined.

The term bandwidth can also be applied to cabling and equipment to give an idea of the range of frequencies that it can transmit or process without undue loss

A higher data rate for signals requires a higher bandwidth to carry it effectively. For transmission media, the higher its bandwidth, the higher total data rate can be carried.

A given bandwidth can support various data rates given the requirements of the signal. The more limited the bandwidth, the greater the distortion at the receiver of the transmitted signal.

If the data rate of the input signal is A bits per second (bps) then a good approximation of the signal can be achieved over a bandwidth of 2A Hz. (Nyquist)

Types of transmission media.

This can be divided into two main groups, guided transmission lines (such as cables)where there is a physical medium within which the signals are carried and unguided transmission lines where there is no visible tangible signal path e.g. a microwave link.

Diagram of bandwidths of various transmission media

Guided transmission lines

Twisted pair

Twisted pair lines are two insulated copper cables twisted together in a spiral. This helps to minimise electromagnetic interference (EMI) between the two wires, sometimes known as coupling. The TP lines were originally designed to carry voice communications only. The human voice occupies a frequency range about 20 Hz to 16 kHz. The telephone system uses 300 to 3400 Hz., i.e. a bandwidth of 3100 kHz. Thus voices can be easily understood but there is a loss of timbre (or quality).

Pairs may be bundled together into cables containing hundreds of pairs. Typical thickness of the individual cables ranges from 0.04 to 0.9 mm. This is the most common transmission medium used for telephony and digital signaling. Telephone standard cable will support 64 Kbps. In a local area network twisted pair is also used (category 5) and data transmission rates of 100 Mbps are common but are limited to station number and length. Long distance dedicated twisted pair can support 4 Mbps.

Twisted pair is cheap and easy to work with, but limited in terms of data transmission rate and length. Couples well so picks up external signals. Different twisting reduces cross-coupling. Digital signals need repeaters every 2-3 km.

Coaxial cable

This consists of two conductors, an outer shielding cable within which is a hollow insulating dielectric shield around a single inner cable. This helps increase the range of frequencies over which it can operate. Diameter can range from 10 to 25 mm. Coaxial cable enjoys widespread use in LANs and short run computer links. It can support a large number of data types and equipment within a local site. Can also support high speed I/O channels on computer systems. Using FDM (frequency division multiplexing) it can carry many signals simultaneously.

Coaxial cable can be used up to several MHz so can support higher data transmission rates over long distances than twisted pair and due to the shielding it is much less susceptible to coupling. For digital signals repeaters every km or less when supporting higher data rates.

Fibre optic

Fibre optic consists of thin strands of transparent media that are capable of transmitting an optical ray. Fused silica is best but expensive. Multicomponent glass fibres are commonly used, each strand being clad with a material of different refractive index. Light is kept within the fibres by the effect of TIR, total internal reflection that happens at the interface of two materials of differing refractive indices. These fibres can be bundled together and covered with a protective sleeve. A recent trial in California has achieved a communication rate of 80 Gbit/ sec using a technique called DWDM.

Fibre optic is smaller and lighter than coax. It displays lower attenuation than coax or twisted pair. It is also unsusceptible to external electromagnetic interference. However, it is difficult to join and does not bend to small radii.

The cost of fibre optic cabling has dropped by more than tenfold in last 15 years and capacity has increased by around the same factor. It has good security characteristics, and needs to be bent to bleed information from itself. It is now used in most long distance land based telephony not being a lossy medium to transfer information. Repeaters can be up to ~100 km apart. In a few years fibre optics will be the dominant medium for fixed installations.

Light for slower links is supplied by LEDs (light emitting diodes) and for faster links ILDs (injection laser diodes) are used, the latter being faster, more expensive but less robust. In optical fibres, light travels best in three distinct "windows" centred on 850, 1300 and 1550 nm. Local applications tend to use 850nm. This is relatively cheap and limited to data rates of <100 Mbps and distances of a few km. For longer wavelengths (i.e. higher frequencies) laser sources are required and higher data rates and distances are achieved.

Unguided transmission methods

Wireless

Satellite and radio links are used for such diverse things as global Positioning Systems (GPS), laptop links and remote access of data, television, mobile telephones, and Internet communications.

Microwave

This method of communication uses highly directional, high frequency (short wavelength) radio waves which are beamed between parabolic dishes situated in line of sight positions. Needs far fewer repeaters than coax. It is also used for short-range communications as data links between LANs. Transmission

frequencies in the range of 2-40 GHz are common. With increased frequency comes increased data transmission rates. Main source of loss is attenuation, rainfall becoming noticeable over 10 GHz. Interference from adjacent transmission areas can occur.

Satellite microwave

This technology uses essentially the same principle as terrestrial microwave with a geostationary satellite above the Earth. The satellite receives transmissions from the ground on the uplink and amplifies and re-transmits these signals earthwards on the downlink. Each satellite operates on a number of frequency bands known as transponder channels.

Satellites may be used for point to point links and for general broadcasting. VSAT (very small aperture terminal) system has been developed and LANs can access each other with suitable equipment at data rates of 256 Kbps or more. The useful frequency range for satellite microwaves is 1-10 GHz. Below this range, the signal is interfered with and above it is attenuated. Most point to point links use the 4/6 GHz band, 6 up and 4 down. This has become saturated so 12/14 GHz band has been developed and also 19/29 GHz which by virtue of higher frequency and therefore lower wavelength allows for smaller and cheaper receivers.

Transmission Techniques

Serial and Parallel Transmission

It is possible to have many lines in the data path between units. This is called parallel transmission. This type of transmission is normally used over very short distances (a few tens of metres). It is costly and unreliable over longer distances due to the differing characteristics of each line in the cable. When the byte(s) arrive at their destination, if they have travelled too far, the separate bits will arrive at different times and the content of the byte(s) will be lost.

Over longer distances, a single line is used. This is called serial transmission.

There are many ways of transmitting digital information through a medium. The normal way to transmit information through a medium is to vary an electric signal at the transmitting end by some means and these variations are detected at the receiver. The amount of information which can be sent through a cable depends on the width of the transmission frequency range known as the bandwidth. For a transmission medium having a particular bandwidth there are a variety of ways of transmitting information through it, baseband and broadband.

Baseband Transmission

In baseband transmission, the voltage encoded signal is applied directly to the medium as a digital pulse (or stream of pulses). The entire frequency spectrum of the cable is used to carry the signal. As the signal passes through the medium is attenuated (i.e. its voltage drops between the sending end and the receiving end) and the quality of the received signal decreases with the distance that it has travelled. In baseband communications, if long distances are involved, some form of amplifier must be inserted into the medium. To make efficient use of this medium, it requires a form of time division multiplexing to squeeze many signals from many computers onto one cable. This can be achieved using either synchronous time division multiplexing or statistical time division multiplexing.

Broadband Transmission

This type of transmission makes use of analogue signaling and frequency division multiplexing, FDM. In order to make more efficient use of the cable, it is possible to divide its bandwidth up into channels. For example, a coaxial cable can accommodate frequencies between 10 MHz and 300 MHz. This may be split into 29 bands of 10 MHz each.

Overview of multiplexing

When an expensive leased line is used, it is advantageous to use as much of its bandwidth as possible. To efficiently use a transmission medium, the technique of multiplexing is used.

Figure 4.1 The technique of multiplexing

It can be seen that many separate input signals are multiplexed onto the high capacity link and then recovered and separated at the far end of the link using a multiplexer.

Conclusion

We have seen that signals may be carried by analogue and digital methods and seen how noise affects their recovery at the remote end. We compared leased lines with dial-up lines. We have seen the concept of bandwidth and then looked at the bandwidths of various transmission media in common use, namely TP, coax, FO, wireless and microwave.

We saw the differences between serial and parallel transmission schemes and looked briefly at the differences between baseband and broadband signaling.

Finally we introduced the concept of multiplexing.

References.

Data and Computer Communications 4th edition. Stallings, Macmillan.

The PC Support Handbook, David Dick, Dumbreck Publishing